1. Field of the Invention
The present invention relates to a temperature control technique in a fixing unit for heating and fixing a toner image onto a sheet.
2. Description of the Related Art
As a temperature control technique for a fixing unit provided in an image forming apparatus, there is known an intermittent control method. The intermittent control method performs the temperature control by turning ON and OFF a heater intermittently to achieve an image fixable temperature in a normal fixing operation. Further, this intermittent temperature control is performed in the case where a predetermined time period has elapsed since a temperature change due to influence of sheet feed operation was recognized or based on a comparison result between a measured temperature and fixing temperature (refer to, e.g., Jpn. Pat. Appln. Laid-Open Publication No. 7-114289).
The voltage of an input power source to be supplied to an image forming apparatus varies depending on the power source environment of the image forming apparatus itself or present/absence of optional apparatus attached to the image forming apparatus (e.g., about ±10%).
It is possible to employ a configuration that detects presence/absence of the optional apparatus attached to the image forming apparatus so as to set the output (W) of a heater. In this case, however, the temperature of a heating roller in a fixing unit varies in accordance with a variation in the input voltage applied to the image forming apparatus.
Typically, in the case where the voltage value of the input power source to the image forming apparatus is significantly decreased, the output of the heater becomes small to cause a disadvantage in the fixing performance. Thus, in many image forming apparatus, the fixing temperature is fixedly set so that the disadvantage in the fixing performance does not occur even at a lowest voltage value of the input power source. However, in the case where the voltage value of the input power source to be supplied to the image forming apparatus is increased, the output of the heater may become too high. Accordingly, the heat to be applied to a sheet becomes too high with the result that curling of the sheet occurs.
As described above, the fixing temperature which is a target temperature of the heating roller in the fixing unit is fixedly set in the above prior art, making it difficult to flexibly respond to a variation in the voltage value of a power source in the image forming apparatus with the result that disadvantages such as defective fixing or curling of a sheet occur.